Donor–acceptor polymers based on multi-fused heptacyclic structures:
synthesis, characterization and photovoltaic applicationsw
Jhong-Sian Wu, Yen-Ju Cheng,* Martin Dubosc, Chao-Hsiang Hsieh, Chin-Yen Chang and
Chain-Shu Hsu*
Received 12th February 2010, Accepted 1st April 2010
First published as an Advance Article on the web 14th April 2010 DOI: 10.1039/c003040f
We report here two novel 2,7-fluorene- and 2,7-carbazole-based conjugated polymers PFDCTBT and PCDCTBT containing
ladder-type heptacyclic structures with forced planarity.
PCDTBT shows excellent solubility, low band gap and high hole mobility, leading to a power conversion efficiency of 3.7%. Over the past few years, tremendous research effort has been made on all-solution processed polymer solar cells (PSCs) in order to realize low-cost, light-weight, large-area and flexible photovoltaic devices. PSCs based on the concept of bulk heterojunction (BHJ) are the most widely adopted device architecture to ensure maximum internal donor–acceptor
(D–A) interfacial area for efficient charge separation.1 To
achieve high efficiency of PSCs, the most critical challenge at the molecular level is to develop p-type conjugated polymers that simultaneously possess (1) sufficient solubility to guarantee solution processability and miscibility with an n-type material, (2) low band gap (LBG) for strong and broad absorption spectrum to capture more solar photons and (3) high hole mobility for efficient charge transport. The most effective approach to produce a LBG polymer is to incorporate electron-rich donor and electron-deficient acceptor segments along the
conjugated polymer backbone.2 Tricyclic 2,7-fluorene3 and
2,7-carbazole4units have emerged as promising electron-rich
building blocks to construct D–A polymers because their derivatives are shown to have deep-lying HOMO energy levels and good hole-transporting properties which are crucial pre-requisites to achieve high open-circuit voltages (Voc) and short
circuit currents (Jsc), respectively. Besides, the electron-rich
thiophene unit is the most important key element ubiquitously incorporated into D–A conjugated polymers to adjust the
optical and electronic properties to optimal levels.5For instance,
alternating copolymers
poly(2,7-fluorene-alt-dithienyl-benzothiadiazole) (PFDTBT)6 and
poly(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT)7have been proven to
be a promising class of p-type photoactive materials for application in PSCs (Scheme 1). In addition to the D–A strategy, forced planarization by covalently fastening adjacent aromatic units in the polymer backbone strengthens the parallel p-orbital interactions to elongate effective conjugation length and facilitate p-electron delocalization, providing an
effective way to reduce the band gap.8 Moreover, coplanar
geometries and rigid structures can suppress the rotational disorder around interannular single bonds and lower the reorganization energy, which in turn enhances the intrinsic
charge mobility.9However, too strong inter-chain p–p stacking
interactions arising from the high degree of coplanarity tend to make the polyaromatic conjugated polymers insoluble and unprocessable. In this communication, we wish to report two novel D–A copolymers poly(fluorene-dicyclopentathiophene-alt-benzothiadiazole) (PFDCTBT) and poly(carbazole-dicyclo-pentathiophene-alt-benzothiadiazole) (PCDCTBT). Both polymers were designed based on the skeletons of the well-known PFDTBT and PCDTBT polymers in order to fully take advantage of their excellent properties. The structural unique-ness of PFDCTBT and PCDCTBT is that the 3-positions of two outer thiophenes are covalently tied with the 3,6-position of central fluorene or carbazole cores by a carbon bridge, forming two cyclopentadienyl (CP) rings embedded in a multi-fused heptacyclic structure (Scheme 1). Two additional substi-tuents attached at the CP rings allow for tailoring the intermolecular interactions without causing twisting between the adjacent units, making the resulting polymers highly soluble. A preliminary test of the photovoltaic performance based on these polymers shows promise for solar cell applications.
The synthesis of the monomers M1 and M2 is depicted in Scheme 2. 2,7-Diboronic esters fluorene 1a and carbazole 1b were reacted with ethyl 2-bromothiophene-3-carboxylate 2 by Suzuki coupling to obtain compounds 3a and 3b respectively. Double nucleophilic addition of freshly prepared 4-(2-ethyl-hexyloxy)phenyl magnesium bromide 4 to the ester groups of 3 led to the formation of benzylic alcohol 5 which was subjected to intramolecular annulation through acid-mediated
Scheme 1 Chemical structures of non-fused PFDTBT, PCDTBT polymers and fused PFDCTBT, PCDCTBT polymers.
Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan.
E-mail: yjcheng@mail.nctu.edu.tw, cshsu@mail.nctu.edu.tw
w Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c003040f
This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 3259–3261 | 3259
COMMUNICATION www.rsc.org/chemcomm | ChemComm
Published on 14 April 2010. Downloaded by National Chiao Tung University on 25/04/2014 06:47:15.
Friedel–Crafts reaction to furnish the fused heptacyclic arene 6. Because of the strong nitrogen-directing effect of the carbazole to facilitate the regioselective cyclization, 6b was obtained in nearly quantitative yield of 97% in the presence of
AcOH, whereas using stronger acids H2SO4/AcOH was required
to generate 6a in a moderate yield of 52%. Carbazole-based 6b can be efficiently lithiated by t-butyllithium through deproto-nation followed by reacting with trimethyltin chloride to afford the distannyl M2. However, fluorene-based 6a was brominated to yield compound 7 which was then lithiated to obtain distannyl M1. M1 and M2 were copolymerized with the acceptor 4,7-dibromo-2,1,3-benzothiadiazole 8 by Stille
coupling to give PFDCTBT (Mn= 8.5 kDa, PDI = 1.89) and
PCDCTBT (Mn = 8.2 kDa, PDI = 1.59), respectively.
Thanks to the branched 2-ethylhexyloxy side chains as the solubilizing groups, both polymers show excellent solubilities in common organic solvents, such as chloroform, toluene, dichlorobenzene, overcoming the planarity–solubility tradeoff to facilitate their processability.
The thermal stability was analyzed by thermogravimetric analysis (TGA). PFDCTBT and PCDCTBT exhibited
suffi-ciently high decomposition temperatures (Td) of 410 1C and
434 1C, respectively, indicating that carbazole-based
PCDCTBT is thermally more stable than fluorene-based PFDCTBT (Fig. S1, ESIw).
Both PFDCTBT and PCDCTBT exhibited two charac-teristic bands in the absorption spectra (Fig. 1). The shorter wavelength absorbance comes from the p–p* transition of the heptacyclic units, while the lower energy band is attributed to the intramolecular charge transfer (ICT) between the electron-rich and the benzothiadiazole segments. Compared to PFDCTBT showing the absorption maxima at 411 nm and 580 nm in the thin film, PCDCTBT exhibited a similar absorption maximum at 412 nm but a bathochromic shift of the ICT band at 607 nm. In addition, the optical band gaps
(Egopt) deduced from the onset of absorption in the solid
state are determined to be 1.76 eV (703 nm) for PFDCTBT
and 1.66 eV (746 nm) for PCDCTBT. These results suggest that the donating strength of carbazole is stronger than that of the fluorene moiety, shifting the ICT band of PCDCTBT to lower energy. It should be emphasized that PFDCTBT and PCDCTBT have more red-shifted absorption spectra and smaller band gaps in comparison with their corresponding
non-fused PFDTBT (Egopt = 1.87 eV) and PCDTBT
(Egopt= 1.88 eV) analogues, demonstrating that the electron
coupling between the rigidified donor and the acceptor units is enhanced. Because two 4-(2-ethylhexyloxy)phenyl moieties substituted at the carbon of CP rings may dilute strong intermolecular p–p interactions, the profiles of absorp-tion spectra of PFDCTBT and PCDCTBT are essentially unchanged with slight broadening of the bands and red shift of the band edges from the solution state to the solid state. This result implies the amorphous nature of the polymers which can be further confirmed by no obvious thermal transi-tion in the differential scanning calorimetry measurements (Fig. S2, ESIw). It is worth noting that the intensities of the shorter wavelength bands of both polymers in the solid state are apparently stronger than those in the solution state, which also suggests that the rigid and coplanar heptacyclic units can enhance their light absorption ability in the solid state.
Cyclic voltammetry (CV) was employed to examine the electrochemical properties (Fig. 2). Both polymers showed a stable and reversible p-doping/n-doping process in the cathodic and anodic scans. The HOMO levels being estimated
to be 5.32 eV for PFDCTBT and 5.38 eV for PCDCTBT
are in an ideal range to assure better air-stability and greater
attainable Vocin the final device.10The LUMO energy levels
are approximately located at 3.55 eV for PFDCTBT and
3.61 eV for PCDCTBT, which are positioned 0.2–0.3 eV
above the LUMO level of the PC71BM acceptor (3.8 eV) to
ensure energetically favorable electron transfer.11This can be
unambiguously evidenced by the complete photoluminescence quenching in the film of the PFDCTBT/PC71BM and PCDCTBT/PC71BM (1 : 2, w/w) blends (Fig. S3, ESIw).
Despite their amorphous nature, PFDCTBT and
PCDCTBT show good hole transporting properties due to their rigid and coplanar structures. Based on the space-charge-limited current (SCLC) method, PFDCTBT and PCDCTBT exhibited high hole mobilities within the same order of magnitude
(2.5 10 4cm2/Vs and 1 10 4cm2/Vs, respectively). On
the basis of ITO/PEDOT:PSS/polymer:PC71BM(1 : 2, w/w)/
Ca/Al configuration, bulk heterojunction solar cells were
fabricated and characterized under simulated 100 mW cm 2
AM 1.5 G illumination. The current density–voltage charac-teristics of the devices are shown in Fig. 3. Without extensive
Scheme 2 Synthetic route of the M1 and M2 monomers leading to the targeted polymers PFDCTBT and PCDCTBT.
Fig. 1 Normalized absorption spectra of PFDCTBT (a) and PCDCTBT (b) in toluene solution and the solid state.
3260 | Chem. Commun., 2010, 46, 3259–3261 This journal is c The Royal Society of Chemistry 2010
Published on 14 April 2010. Downloaded by National Chiao Tung University on 25/04/2014 06:47:15.
optimization, the preliminary photovoltaic performance based
on PFDCTBT already showed a Jscof 9.5 mA cm 2, a Vocof
0.77 V, a fill factor (FF) of 0.38, leading to a decent PCE of 2.8%. More encouragingly, the device using PCDCTBT as the
p-type material delivered superior performance with a Jscof
10.7 mA cm 2, a V
ocof 0.80 V, a FF of 0.43, improving the
PCE to 3.7%.
To further evaluate the hole mobility in the BHJ active layer, hole-only devices (ITO/PEDOT:PSS/polymer:PC71BM (1 : 2, w/w)/Au) were fabricated. It is found that the hole mobility
of the PCDCTBT/PC71BM composite (4 10
4
cm2/Vs) is
higher than that of the PFDCTBT/PC71BM (5 10 5cm2/Vs)
blend under the same fabrication conditions. The enhanced hole mobility of the PCDCTBT/PC71BM active layer might be responsible for its better photovoltaic performance over the PFDCTBT-based device. It is also noteworthy that the surface
roughness of the PCDCTBT/PC71BM blend observed by
AFM is larger than that of the PFDCTBT/PC71BM blend (Fig. S4, ESIw).
In summary, by utilization of facile Friedel–Crafts cycli-zation, we have successfully synthesized two well-designed heptacyclic monomers M1 and M2 in which two outer thiophene subunits are covalently fastened to the central 2,7-fluorene and 2,7-carbazole cores, respectively. Rigid, coplanar and rich M1 and M2 were copolymerized with the electron-deficient benzothiadiazole acceptor by Stille coupling to afford two novel D–A polymers PFDCTBT and PCDCTBT, respec-tively. Through such a simple and straightforward engineering of molecular structures, PFDCTBT and PCDCTBT simul-taneously possess excellent solubilities for solution-processability, low band gaps with suitable position of HOMO/LUMO energy levels, and high hole mobilities, leading to promising
PCEs of 2.8% and 3.7%, respectively. We anticipate that further improvement of device performance is highly achiev-able through carefully optimizing the processing conditions which are ongoing in our laboratory.
This work is supported by the National Science Council and ‘‘Aim for the Top University Plan’’ of the National Chiao Tung University and Ministry of Education, Taiwan.
Notes and references
1 (a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789; (b) S. Gu¨nes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 107, 1324; (c) B. C. Thompson and J. M. J. Fre´chet, Angew. Chem., Int. Ed., 2008, 47, 58; (d) Y.-J. Cheng, S.-H. Yang and C.-S. Hsu, Chem. Rev., 2009, 109, 5868.
2 (a) J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6, 497; (b) E. Zhou, M. Nakamura, T. Nishizawa, Y. Zhang, Q. Wei, K. Tajima, C. Yang and K. Hashimoto, Macromolecules, 2008, 41, 8302; (c) J. Hou, H.-Y. Chen, S. Zhang, G. Li and Y. Yang, J. Am. Chem. Soc., 2008, 130, 16144.
3 (a) G. L. Schulz, X. Chen and S. Holdcroft, Appl. Phys. Lett., 2009, 94, 023302; (b) F. Zhang, W. Mammo, L. M. Andersson, S. Admassie, M. R. Andersson and O. Ingana¨s, Adv. Mater., 2006, 18, 2169.
4 (a) J. Li, F. Dierschke, J. Wu, A. C. Grimsdale and K. Mu¨llen, J. Mater. Chem., 2006, 16, 96; (b) N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-Plesu, M. Belleteˆte, G. Durocher, Y. Tao and M. Leclerc, J. Am. Chem. Soc., 2008, 130, 732.
5 (a) E. Wang, M. Wang, L. Wang, C. Duan, J. Zhang, W. Cai, C. He, H. Wu and Y. Cao, Macromolecules, 2009, 42, 4410; (b) P. M. Beaujuge, W. Pisula, H. N. Tsao, S. Ellinger, K. Mu¨llen and J. R. Reynolds, J. Am. Chem. Soc., 2009, 131, 7514. 6 (a) M. Svensson, F. Zhang, S. C. Veenstra, W. J. H. Verhees, J. C. Hummelen, J. M. Kroon, O. Ingana¨s and M. R. Andersson, Adv. Mater., 2003, 15, 988; (b) Q. Zhou, Q. Hou, L. Zheng, X. Deng, G. Yu and Y. Cao, Appl. Phys. Lett., 2004, 84, 1653; (c) F. Zhang, K. G. Jespersen, C. Bjo¨rstro¨m, M. Svensson, M. R. Andersson, V. Sundstro¨m, K. Magnusson, E. Moons, A. Yartsev and O. Ingana¨s, Adv. Funct. Mater., 2006, 16, 667; (d) L. H. Slooff, S. C. Veenstra, J. M. Kroon, D. J. D. Moet, J. Sweelssen and M. M. Koetse, Appl. Phys. Lett., 2007, 90, 143506.
7 (a) N. Blouin, A. Michaud and M. Leclerc, Adv. Mater., 2007, 19, 2295; (b) S. Wakim, S. Beaupre´, N. Blouin, B.-R. Aich, S. Rodman, R. Gaudiana, Y. Tao and M. Leclerc, J. Mater. Chem., 2009, 19, 5351; (c) T.-Y. Chu, S. Alem, P. G. Verly, S. Wakim, J. Lu, Y. Tao, S. Beaupre´, M. Leclerc, F. Be´langer, D. De´silets, S. Rodman, D. Waller and R. Gaudiana, Appl. Phys. Lett., 2009, 95, 063304; (d) S. H. Park, A. Roy, S. Beaupre´, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics, 2009, 3, 297.
8 (a) J. Roncali, Macromol. Rapid Commun., 2007, 28, 1761; (b) C.-P. Chen, S.-H. Chan, T.-C. Chao, C. Ting and B.-T. Ko, J. Am. Chem. Soc., 2008, 130, 12828; (c) Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Mu¨hlbacher, M. Scharber and C. Brabec, Macromolecules, 2007, 40, 1981.
9 (a) J. L. Bre´das, J. P. Calbert, D. A. da Silva Filho and J. Cornil, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 5804; (b) S. Ando, J.-I. Nishida, H. Tada, Y. Inoue, S. Tokito and Y. Yamashita, J. Am. Chem. Soc., 2005, 127, 5336; (c) N.-S. Baek, S. K. Hau, H.-L. Yip, O. Acton, K.-S. Chen and A. K.-Y. Jen, Chem. Mater., 2008, 20, 5734; (d) Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li and L. Yu, J. Am. Chem. Soc., 2009, 131, 56.
10 C. J. Brabec, A. Cravino, D. Meissner, N. S. Sariciftci, T. Fromherz, M. T. Rispens, L. Sanchez and J. C. Hummelen, Adv. Funct. Mater., 2001, 11, 374.
11 J. J. M. Halls, J. Cornil, D. A. dos Santos, R. Silbey, D.-H. Hwang, A. B. Holmes, J. L. Bre´das and R. H. Friend, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 60, 5721.
Fig. 2 Cyclic voltammograms of PFDCTBT and PCDCTBT in the thin film at a scan rate of 30 mV s 1.
Fig. 3 Current density–voltage characteristics of ITO/PEDOT: PSS/polymer:PC71BM/Ca/Al devices under illumination of AM
1.5 G, 100 mW cm 2.
This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 3259–3261 | 3261
Published on 14 April 2010. Downloaded by National Chiao Tung University on 25/04/2014 06:47:15.
